Page 107 



CONTROL AND SIGNAL BUILDING 



2462 



Figure 18. — Sextant location of stations along a precipitous shore. 



In the figure, the three triangulation stations X, Y, and Z were established from landward and cannot be seen from the shore 

 immediately below, although they are visible from the water area a short distance offshore. After whitewashes have been placed or 

 other objects selected along the shore, represented by points ^ to H iaclusive, the survey vessel occupies successive ship stations, indi- 

 cated on the sketch by 1 to 5 inclusive, which are located by three-point fixes observed to the triangulation stations X, Y, and Z. 

 The vessel should preferably be anchored at each station. (See 2432.) As each successive ship station is located, simultaneous sextant 

 angles are observed to the new stations along the beach. The angles should be measured from an initial station so located that they 

 will be nearly horizontal, and if any of them are inclined appreciably they should be corrected accordmg to 3338. 



If the topography of the shorehne along such a coast cannot be surveyed in any 

 other manner, it may be sketched with reference to the whitewashes or the buoys from 

 a launch or small boat using a sextant to cut in other points. 



2462. Control in Narrow Unimportant Waterways 



It is frequently impracticable to locate control by conventional methods up 

 narrow unimportant sloughs or creeks, especially where the shores of these are frmged 

 by mangroves or other vegetation which prevents landing on them. Waterways with 

 sufficient depth of water, or important enough to be useful in navigation, require accurate 

 detailed surveys and the control in them must be established by conventional methods 

 regardless of effort or cost. In sloughs through mangroves or in swamps or minor 

 tributaries less than 200 meters in width, where the depth of water is trivial and there 

 is no known navigation of importance, less accurate methods can be tolerated where 

 control by more conventional methods cannot be established economically. If the 

 vegetation on the banks is not too tall it is frequently possible to control the hydro- 

 graphic survey from a few accurately located points at some distance from the shore, 

 by observations made over the vegetation without the necessity of having control 

 points on the shores themselves. Wliere the waterway is shut in by trees or other 

 vegetation on both banks so that nothing beyond them can be seen, an awkward problem 

 is presented. One method of locating the control is the sextometer method, described 

 in 2382. 



If the waterway is short and not too narrow the control may be located by a sex- 

 tant triangulation up the stream. It is rarely possible in such cases to arrange well- 

 conditioned figures and the surveyor must frequently resort to the use of single triangles, 

 often without adequate checks. The signals to be used for control should be built 



